1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Description of the Related Art
Conventionally, image processing methods and apparatuses are discussed to improve diagnostic performance in a radiographic image by frequency processing. In the frequency processing, a plurality of band limit images (band limit signals) indicating frequency components of limited frequency bands are generated from an original image before frequency emphasis and every band limit image is emphasized or suppressed. A method for generating a plurality of band limit images includes a method using Laplacian pyramid decomposition, a method using wavelet transformation, a method using an unsharp mask, and the like. If the unsharp mask is used, a band limit image HLv is expressed with an original image Sorg and a blurred image SusLv as the following Expression 1.[Expression 1]HLv(x,y)=Sorg(x,y)−SUsLv(x,y)  (1)
An index Lv is of a band limit image. Blurred images with different frequency response characteristics are generated, and various band limit images can be obtained. When using an image L at the lowest frequency with band limit, a relation between a band limit image and the original image is expressed as the following Expression 2.
                    [                  Expression          ⁢                                          ⁢          2                ]                                                                                  S            org                    ⁡                      (                          x              ,              y                        )                          =                                            ∑                              Lv                =                1                            LvMax                        ⁢                                          H                Lv                            ⁡                              (                                  x                  ,                  y                                )                                              +                      L            ⁡                          (                              x                ,                y                            )                                                          (        2        )            
The decomposed band limit images are added, and the original image is restructured. When using an amplitude response F (HLv) at the frequency of each the band limit images, an amplitude response F of an original image (Sorg) can be expressed as the sum of amplitude responses of the band limit images in Expression 3.
                    [                  Expression          ⁢                                          ⁢          3                ]                                                                      F          ⁡                      (                                          S                org                            ⁢              org                        )                          =                                            ∑                              Lv                =                1                            LvMax                        ⁢                          F              ⁡                              (                                  H                  Lv                                )                                              +                      F            ⁡                          (              L              )                                                          (        3        )            
Desired frequency response characteristics can be generated by adjusting the amplitude response F for each band limit image.
The frequency processing is described using the following Expression 4 with a band limit image H and an index Lv of the band limit image H. A coefficient α (≧0) is used for emphasizing the band limit image H. A value β (≧0) is used for adjusting an emphasis degree.[Expression 4]H′Lv(x,y)=HLv(x,y)+β×(αLv−1)×HLv(x,y)  (4)
In Expression 4, a balance of the basic frequency response characteristics is defined with a value of the emphasis coefficient α for each band limit image, and the degree of effect of the balance is controlled by the emphasis degree β. When β=1, a relation between the emphasis coefficient α and the band limit image H is as follows. If α=1, the band limit image H is output without any processing. If α>1, the band limit image H is emphasized and output. If α<1, the band limit image H is suppressed and output.
Japanese Patent No. 3696339 discusses, as a method for adjusting frequency response characteristics, a method for plotting the frequency response characteristics on a graphic user interface (GUI) by a user and obtaining a parameter necessary for frequency emphasis processing from the plotting result.
However, if the frequency characteristics are freely determined, an unexpected artifact can be generated depending on frequency characteristics. For example, with respect to a parameter obtained from a certain frequency characteristics, β×(α−1)<−1 is obtained in Expression 4, and a phase of an output image can be inverted and a phase inversion artifact may be generated.